Engines may use a turbocharger to improve engine torque/power output density. In one example, a turbocharger may include a compressor and a turbine connected by a common shaft, where the turbine is coupled to the exhaust manifold side and the compressor is coupled to the intake manifold side. In this way, the exhaust-driven turbine supplies energy to the compressor to increase the flow of air into the engine.
In some conditions, turbocharged diesel engines may experience a phenomenon known as “surge.” For example, during a heavy driver tip-out, the engine may slow down while the turbocharger continues to spin for some time before it gradually slows down. This delay may be at least partially due to inertia and continuing power input from the exhaust turbine. The engine slow down causes a continuing reduction of air flow through engine. Further, the continuing action of the turbocharger can cause rapid pressure build-up on the compressor outlet or intake manifold side and rapid pressure reduction/drain on the turbine or exhaust manifold side. Compressors surge may occur when forward flow through the compressor can no longer be sustainable, due to an increase in pressure across the compressor, and a momentary flow reversal occurs. Once surge occurs, the reversal of flow reduces the discharge pressure or increases the suction pressure, thus allowing forward flow to resume again until the pressure rise again reaches the surge point. Such flow instability and the resultant noise can be referred to as “surge.”
One approach to address surging is described in U.S. Pat. No. 6,725,660. In the '660 reference, a control action that temporarily increases the opening of nozzle vanes located before the turbine inlet is performed immediately after deceleration. Allegedly, the flow velocity of the exhaust striking the turbine is abruptly reduced so that a state resembling the application of a braking force to the turbine is produced. This braking force is used to abruptly decrease the rotational speeds of the turbine and compressor. As a result, the compressor pressure ratio can be reduced, so that surging can be prevented.
However, the inventors herein have recognized that this approach may provide degraded results under some conditions and may still result in surge. For example, under some conditions, high levels of EGR may be used to decrease NOx emissions. The high EGR levels can then push the engine breathing line closer to a surge line even at steady state since turbochargers are typically matched to meet low engine speed and full load torque requirements close to the surge line. During heavy engine tip-outs, the engine decelerates, while turbochargers, due to their inertia and continuous power input from turbines, continue to spin before they gradually slow down Thus, even with increased braking of the turbine, the pressure at the compressor outlet side may not be released to a level low enough to prevent compressor surge.
Another approach that attempts to prevent surging is described in U.S. Patent Application No. 2004/0244375. The '375 reference shows an intake air release means that opens an EGR valve and causes part of the intake air present in intake passage to flow into the exhaust passage via EGR passage, thereby reducing the intake pressure inside the intake when the vehicle is decelerating.
Again, the inventors herein have recognized a problem with such an approach. For example, under some conditions, the pressure expansion ratio across the turbine can be relatively high (due to flow resistance in the turbine). In other words, the pressure at turbine inlet, or the pressure of exhaust manifold, is relatively high. Thus, only a small amount of EGR can flow to the engine exhaust side. As a result, the intake manifold pressure, or the pressure of the compressor outlet, may not decrease enough to prevent the flow reversal (surge) in the compressor under some conditions.